Add docs snippet validation gate

[bfernando]

Summary

• add a Node/TypeScript snippet checker that extracts TS/JS MDX fences and compiles them with tsc --noEmit
• add a GitHub Actions workflow for snippet checks on PRs and pushes to develop/main
• add a non-blocking Stellar testnet validation job that verifies five Horizon response shapes
• document the snippet workflow and no-check opt-out in docs/CONTRIBUTING.md
• mark existing illustrative schema/service fragments as no-check

Notes

The first CI gate is intentionally syntax-focused because the current docs contain many illustrative fragments and API-shape examples that are not standalone programs. The Stellar job is non-blocking because it depends on live testnet/network availability.

Validation

• npm test
• npm run check:stellar-testnet
• npx tsc --noEmit

Fixes #27

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[truthixify]

Merge is clean. The snippet validator + testnet variant + CI workflow + CONTRIBUTING update all look good.

One blocker: drop package-lock.json (2184 lines, npm). This repo uses pnpm.

rm package-lock.json
git add -u
git commit --amend
git push --force-with-lease

After that I will merge.

[truthixify]

Test-merges cleanly otherwise. The 2184-line package-lock.json is still in the PR. This repo uses pnpm.

rm package-lock.json
git add -u
git commit --amend --no-edit
git push --force-with-lease

Snippet validators + CI wiring + CONTRIBUTING update all look good once the lockfile is gone.